The understanding of degradation modes and mechanism is very important in order to ensure the lifetime of 25–30years of PV modules. In the present study, degradation analysis of 90 mono-crystalline silicon PV modules installed on the rooftop of the guest house of National Institute of Solar Energy (NISE), Gurgaon has been carried out after 22years of outdoor operation in a composite climate of India. A comprehensive analysis has been carried out through visual inspection, thermal imaging, I-V characteristic and insulation resistance measurement as well as rate of degradation has been calculated. The defects in bus bar, cell inter-connection ribbon, string inter-connection ribbon and chalking in back-sheet are found to be most frequently observed defects. Hot spot in solar cell, burn mark and delamination in back-sheet are also observed in some PV modules. The average power degradation rate of 90 PV modules over period of 22years has been found to be about 1.9%/year with maximum rate of power degradation 4.1%/year and minimum is 0.3%/year. Insulation resistance measurement of all the modules both in dry and wet condition showed that only 9 modules have shown insulation resistance <400MΩ. The study addresses the degradation mechanism in order to assess the safety issues related to the large scale PV plants in Indian climatic condition.